1. Field of the Invention
This invention relates to a method of coating substrates, particularly aggregates for road building, with asphalt. It may also be used to coat other materials such as paper, paperboard (corrugated or otherwise).
2. Description of the Prior Art
Asphalt (which is sometimes referred to as "bitumen") has previously been used for coating aggregates which are to be used in road building, for example, for effecting soil stabilisation. The coating may be carried out in a coating plant or in situ, but in either case, it is difficult to mix the asphalt with the solid aggregate except under special conditions and even then, it is difficult to coat the aggregate evenly when it is cold or moist or both. This often causes trouble because road building aggregate is usually stored out of doors where it easily becomes wet and often cold.
Various proposals have been made for solving these problems but all of them up to the present, have been unsatisfactory in one respect or another.
For example, Csanyi in U.S. Pat. No. 2,917,395 proposes the use of an asphalt foam to coat the aggregate. In principle, this proposal has substantial merit because the foam can be spread more easily than the asphalt itself and because the foam occupies a greater volume than the original asphalt it is relatively easy to spread a smaller amount of asphalt on the aggregate, thus effecting a more economical use of the asphalt binder. The asphalt foam is formed by the use of steam which is mixed with the hot, liquid asphalt in a nozzle to form a foam which can then be applied to an aggregate and mixed with it.
The commercial utilization of this method has, however, been retarded because of the relatively poor and inconsistent quality of the foam it produces. Foamed asphalt is well suited for coating and binding aggregate, even when cold and moist, if it is expanded to a volume of at least 10, and generally 15 to 20 or even 50 times or greater than that of the asphalt from which it is formed but lower volumes become increasingly poorer in their ability to be mixed with aggregate. It is common to obtain a foam:asphalt ratio of as low as 2:1 by this steam foaming method, and in practice it is unlikely that the ratio will rise above 4:1. Practical considerations render it extremely difficult, if not impossible, to mix steam into asphalt or any other relatively viscous material, in amounts such that the desired expansion can be achieved. For example, in order to obtain a foam:asphalt volume ratio of 17:1 at a typical working temperature of 280.degree. F., four volumes of steam would be required for each volume of asphalt at a mixing pressure of 4 atmospheres absolute, and incorporation of this amount of steam is hardly practicable in the time allowed and in the equipment used.
A further disadvantage of the steam foaming method is that it requires a source of water which is suitable for steam production. Yet another disadvantage is the practical difficulty of controlling the foaming operation so as to enable selection of a desired foam:asphalt ratio.
Another solution to the problem was proposed in U.S. Pat. No. 3,423,222 by McConnaughay. In this case the method involves heating the aggregate in a drum and coating the hot aggregate with a cloud of the asphalt binder discharged from a nozzle. The liquid asphalt is mixed with water and discharged in the form of a turbulent dispersion or cloud onto the hot aggregate.
The disadvantage of this method is that it relies upon the use of a heater and this greatly increases the costs of the operation particularly when the aggregate must be heated, as recommended, to a temperature of 400.degree. F. at the point of discharge. In addition, the capital cost of the equipment is high. It would be clearly desirable to eliminate the need for heating the aggregate and for avoiding, if possible, the use of expensive equipment.
Ditto U.S. Pat. No. 2,283,192 discloses a method for mixing bituminous cutbacks and other hydrocarbon oils with aggregates such as crushed stone or coal dust by combining water with the hydrocarbon oil in a colloid mill under pressure. The hydrocarbon oil may be heated prior to being mixed with the water and, if necessary, the emulsion can be heated in the mill. After the water/oil emulsion has been formed it can be subjected to further heating or cooling if desired. The emulsion is then discharged to the atmosphere through a nozzle to form a foam which is applied to the aggregate and mixed with it.
There are a number of disadvantages to the Ditto process. First, it relies upon the use of a mechanical colloid mill to form the emulsion. We have found that the use of such mills is undesirable not only because they are difficult to maintain--especially when attempts are made to employ high melting point asphalts in the process--but also because they produce very fine dispersions which have to be kept under considerable pressure to prevent the water evaporating. When asphalt is used, as the hydrocarbon medium, it generally requires to be at a relatively high temperature; when the water is dispersed into this hot asphalt by the colloid mill the heat transfer to the small water droplets is extremely fast and tends to cause premature evaporation of the water unless the entire system is maintained under a high pressure. This is generally undesirable because the equipment becomes bulky, heavy and costly and the process more difficult to control and operate. In addition, the Ditto process is inefficient in its use of heat: heat may be added to the system after the emulsion is formed, thus compounding the premature evaporation problem or, alternatively, if the action of the mill generates excessive heat--as it frequently does--the emulsion must be cooled, making it less thermally efficient.
It would be desirable to devise a process which is more thermally efficient than the Ditto process and which, moreover, does not require the use of expensive equipment such as colloid mills and pressure vessels.